Organic electronic device

ABSTRACT

This invention generally relates to organic electronic devices and to methods for their fabrication. More particularly we will describe organic thin film transistor (TFT) structures and their fabrication. 
     An organic electronic device, the device comprising: a substrate supporting a first electrode; a spacer structure over said substrate; a second electrode over said spacer structure and at a height above said first electrode; and a layer of organic semiconducting material over said first and second electrodes to provide a conducting channel between said first and second electrodes; and wherein a majority of said first electrode is laterally positioned to one side of said channel and a majority of said second electrode is laterally positioned to the other side of said channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to organic electronic devices and tomethods for their fabrication. More particularly we will describeorganic thin film transistor (TFT) structures and their fabrication.

2. Related Technology

There is a general need for improved organic electronic devices, inparticular thin film transistor devices. One technique for fabricatingvertical-channel polymer field-effect transistors is described in,“Self-aligned vertical-channel, polymer field-effect transistors” by N.Stutzmann, R. H. Friend, and H. Sirringhaus, Science, Vol. 299, 21 Mar.2003, pages 1881-1884). Broadly speaking this describes a technique inwhich layers of the device are micro cut by a sharp wedge to provideaccess from the side to electrically conductive layers. This techniqueis useful for providing a short gate length but a device structure andfabrication method which lends itself more readily to manufacturingwould be beneficial.

Parashkov et al, Appl. Phys. Lett. 82(25), 4759-4580, 2003 discloses avertical-channel thin film transistor wherein a drain electrode isprovided on a substrate; a photoresist is deposited over the drainelectrode; a layer of organic conductive material PEDOT:PSS is depositedover the photoresist; the layer of PEDOT:PSS is patterned to form asource electrode; the layer of photoresist is patterned to expose thedrain electrode; and the organic semiconducting material, gatedielectric and gate electrode are then deposited over the source anddrain electrodes to complete the device. Again, a device structure andfabrication method which lends itself more readily to manufacturing of avertical-channel device would be beneficial.

SUMMARY OF THE INVENTION

According to the present invention there is therefore provided a methodof fabricating an organic thin film transistor on a substrate the methodcomprising:

depositing a separator layer on said substrate;

patterning said separator layer to define a source-drain separator;

depositing conductive material on said substrate bearing saidsource-drain separator to define source and drain electrodes of saidtransistor;

depositing organic semiconducting material over said source and drainelectrodes;

depositing dielectric material over said organic semiconductingmaterial; and

depositing conductive material over said dielectric material to providea gate electrode for said transistor.

Preferably, one of the source and drain electrodes is formed over theseparator structure at a first height, and the other of the source anddrain electrodes is formed over the substrate at a second, lower height.

Preferably, the source and drain electrodes are separated by a distanceof less than 10 μm.

Preferably, the separator layer is formed from photoresist material, andthe separator layer is patterned by photopatterning to form thesource-drain separator.

In one preferred embodiment, the source-drain separator has an undercutedge.

Preferably, the conductive material forming the source and drainelectrodes is deposited by evaporation or sputtering.

Preferably, said forming of said source and drain electrodes comprisesdirectional deposition of electrode material at an angle such that ashadow of an edge of said separator structure defines an electrode edge.

Preferably, the conductive material forming the source and drainelectrodes comprises a metallic element.

Preferably, the conductive material forming the source and drainelectrodes consists essentially of an elemental metal or an alloythereof.

Preferably, at least one of the organic semiconducting material, thedielectric material and the gate electrode are deposited from a solutionin a solvent.

In another aspect, the invention provides an organic electronic device,the device comprising: a substrate supporting a first electrode; aspacer structure over said substrate; a second electrode over saidspacer structure and at a height above said first electrode; and a layerof organic semiconducting material over said first and second electrodesto provide a conducting channel between said first and secondelectrodes; and wherein substantially all the first electrode islaterally positioned to one side of the channel and substantially allthe second electrode is laterally positioned to the other side of thechannel.

Thus, the first and second electrodes are substantially non-overlappinglaterally.

Preferably said height is defined by a thickness of said spacer layer;preferably it is less than 10 μm, 5 μm, 2 μm, or 1 μm. The spacerstructure may comprise a layer of electrically insulating material (inthis specification the terms “insulating” and “dielectric” are usedsynonymously) but, as will be appreciated from the describedembodiments, this is not essential.

The layer of insulating material preferably has a substantially verticalor, more preferably, undercut edge at the channel. The layer ofinsulating material may comprise a layer of resist material which may beetched, for example, anisotropically to produce such an undercut edge.

The organic electronic device may comprise one of a range of devicesincluding, but not limited to, a diode device, a thyristor device, andthe like. However in some particularly preferred embodiments the devicecomprises a transistor, more specifically a field effect transistor suchas a polymer field effect transistor. In this case the first and secondelectrodes provide source and drain electrodes for the transistor andthe device further comprises a layer of gate dielectric over the organicsemiconducting material, more particularly over the channel of thetransistor, and a gate electrode over the gate dielectric. Inembodiments the gate electrode overlies the source and drain electrodeswithout an intervening conducting layer—that is there is no conductinglayer, at least in the vicinity of the channel, between the gateelectrode and the drain electrode and between the gate electrode and thesource electrode.

(The skilled person will understand that references to a conductingchannel are not limited to references to a channel in a field effectdevice and include, for example, a one-way conduction channel of adiode). The skilled person will appreciate that such a field effecttransistor may operate in either enhancement or depletion mode.

In embodiments of the invention a double transistor structure isprovided comprising two transistors each as described above. In such anarrangement one of the source/drain electrodes may be common to both thetransistors; in particular this is the electrode over the spacerstructure. Such a double transistor structure may lack an externalconnection to this common or shared electrode. Such a double transistorstructure can provide improved functionality, such as better switchingisolation.

Organic thin film transistor embodiments of the above-describedstructure provide a number of advantages. One advantage is the extremelyshort gate length which is achievable in such vertical devices ascompared with lateral devices. In a typical lateral device a gate length(a distance between the source drain electrodes) may be of the order of10 μm whereas in structures according to embodiments of the invention agate length of 1 μm, 0.5 μm, 0.2 μm, 0.1 μm or less is readilyachievable. A short gate length provides advantages such as an increasedsource-drain current.

This invention further provides an organic electronic device, the devicecomprising: a substrate; a first electrode over said substrate at afirst height above said substrate; a spacer structure over saidsubstrate; a second electrode over said spacer structure and at a secondheight, greater than said first height, above said substrate; and alayer of organic semiconducting material over said first and secondelectrodes to provide a conducting channel between said first and secondelectrodes; and wherein there is substantially no lateral overlapbetween said first and second electrodes.

The above-mentioned increase source-drain current is particularly usefulfor organic light emitting diode (OLED) displays, especially for drivertransistors of an active matrix OLED display.

Thus in a further aspect the invention provides an active matrix OLEDdisplay, the display having a plurality of pixels, each said pixelhaving associated pixel driver circuitry, and wherein said pixel drivercircuitry includes at least one organic thin film transistor (TFT) inwhich one of a drain and source electrode of said organic TFT isvertically disposed above a substrate of said display at a differentheight to the other of said drain and source electrode of said organicTFT.

A further advantage of the organic TFT structures we describe is thevery high uniformity of gate length which is achievable. For exampleacross a substrate of dimension greater than, say, 5 cm or 10 cm thegate length uniformity may be better than 10%, 5%, 2% or 1%. Expressingthis differently the aforementioned uniformity may be achieved across,for example, greater than 1,000,000 or greater than 10,000,000 organicthin film transistors. Consider, for example, a display comprisingapproximately 500 pixels resolution on each axis (row and column). Ifthe display is a colour display there are 1500 pixels on each axis and,for a typical active matrix driver, 4 transistors per pixel giving 6,000transistors per axis for approximately 36,000,000 transistors over thearea of the display.

Thus an active matrix OLED display as described above may comprisegreater than 1,000,000 or greater than 10,000,000 transistors with agate length uniformity of better than 10%, 5%, 2% or 1%.

In a complementary aspect the invention provides a method of fabricatingan organic electronic device on a substrate, the method comprising:providing said substrate with a first electrode for said organicelectronic device; forming a separator structure on said substrate;forming a second electrode for said organic electronic device over saidseparator structure; and depositing a layer of organic semiconductingmaterial over said first and second electrodes; and wherein saidseparator structure comprises a layer of insulating material, andwherein said forming of said separator structure comprises forming saidsecond electrode over said layer of insulating material and thenpatterning said layer of insulating material using said second electrodeas an etch mask and/or photomask.

As will be described later the different stages of the method may beperformed in a number of different orders, depending upon the embodimentof the method employed.

In embodiments the layer of insulating material may be deliberatelyunder-etched, so that an edge of the separator structure slopes down tothe substrate, more particularly to the first electrode, rather thanhaving a vertical edge. In embodiments the separator structure is formedsuch that it partially overlaps an edge of the first electrode adjacenta channel of the device.

In further embodiments of the method a double TFT structure may befabricated in which the electrode on the separator structure acts as acommon source or drain electrode for the pair of transistors.

In a further related aspect of the invention there is provided a methodof fabricating an organic thin film transistor on a substrate the methodcomprising: depositing a first layer of conductive material on saidsubstrate and patterning said first layer of conductive material todefine a first source or drain electrode of said transistor; depositinga layer of insulating material on said substrate over said firstelectrode; depositing a second layer of conductive material over saidlayer of insulating material and patterning said second layer ofconductive material to define a second, drain or source electrode ofsaid transistor; patterning said layer of insulating material using saidsecond electrode as an etch mask and/or photomask to expose at leastpart of said first electrode; depositing organic semiconducting materialover said first and second electrodes; depositing gate dielectricmaterial over said organic semiconducting material; and depositingconductive material over said dielectric material to provide a gateelectrode for said transistor.

In embodiments of these methods it will be appreciated that thedepositing of the organic semiconductor material over the first andsecond electrodes creates a channel for the transistor, and the gatedielectric is then deposited over this channel, followed by a gateelectrode. Generally vias, connections to other devices and the like mayalso be formed.

Further aspects of the invention provide a device, transistor, anddisplay, in particular OLED display, fabricated using a method asdescribed above.

The skilled person will appreciate that features and aspects of theabove-described structures and methods may be combined in anypermutation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows a first example of a thin film transistor according to anembodiment of the invention;

FIG. 2 shows schematically, some example alternative configurations ofthe edge of the separator structure shown in FIG. 1;

FIGS. 3 a to 3 d schematically illustrate a view from above of examplealternative configurations for the source and drain electrodes of thetransistors of FIGS. 1 and 4 respectively;

FIG. 4 shows a second example of a thin film transistor according to anembodiment of the invention;

FIGS. 5 a to 5 c show, respectively, first and second examples of adouble transistor structure based upon the structures of FIGS. 1 and 4;and a circuit for the double transistor structure of FIG. 5 a/b;

FIGS. 6 a to 6 c show, respectively, a schematic diagram of abottom-emitting OLED display, a vertical cross-section through a portionof a light emitting pixel of the display of FIG. 6 a, and an exampledriver circuit for an active matrix OLED display; and

FIG. 7 shows a view from above of a portion of a complete active matrixOLED display within which a transistor according to an embodiment of theinvention may be incorporated.

DETAILED DESCRIPTION

Referring now to FIG. 1, this shows an example of a preferred embodimentof a thin film transistor device structure 100 according to theinvention. The TFT structure comprises a substrate 102 on which isfabricated a separator structure 104 followed by source and drainelectrodes 106, 108. A layer of organic thin film transistor material110, generally an organic semiconductor such as a polythiophenederivative is then deposited over the source and drain followed by alayer of gate dielectric material 112 and then a gate electrode 114. Inoperation a channel is formed between the edges 106 a, 108 a of thesource and drain electrodes also extending over a part of the topsurface of the source electrode near edge 106 a and also generallyextending over a part of the top surface of the drain electrode nearedge 108 a. Although not shown in FIG. 1 a practical device may alsoconclude one or more vias within the separator structure within theseparator structure 104.

Example materials for the TFT of FIG. 1 are as follows:

Substrate: glass or plastic; a flexible plastic such as polycarbonate orpolyethylene terephthalate (PET).

Separator structure: positive or negative photo resist.

Source/drain: aluminium; a combination of aluminium and chrome layers,for example aluminium sandwiched between layers of chrome. Moregenerally any metal providing an appropriate ohmic connection to theorganic semiconductor. Other examples include gold or palladium for ap-channel device; calcium or barium for an n-channel device.

Organic semiconductor: polythiophene or derivative thereof; polyanilineor derivative thereof; pentacene or derivatives thereof.

Gate dielectric: BCB (Benzocyclobutene); the gate dielectric may also beinorganic, for example SiOx or SiNx.

Gate: as for source/drain; also the gate material may be a polymer suchas Poly(3,4-ethylenedioxythiophene) (PEDOT) or more specificallycomprising poly(styrene sulfonate)—(PEDOT:PSS).

Example layer thicknesses are as follows.

Source/drain: 5 nm-500 nm preferably 10 nm to 150 nm, for exampleapproximately 50 nm (a minimum thickness of 5 nm is generally requiredto achieve electrical continuity).

Organic semiconductor: 50 nm-500 nm, for example 100 nm

Gate dielectric: 50 nm-500 nm, for example 100 nm.

Gate: 5 nm-500 nm, preferably 10 nm to 150 nm, for example approximately50 nm (a minimum thickness of 5 nm is generally required to achieveelectrical continuity).

Separator structure: Preferably the separator has a minimum thickness of50 nm, more preferably a minimum thickness of 100 nm.

The height of the separator structure may be chosen in accordance with adesired gate length for the transistor. For example the height (gatelength) may be less than 10 μm, 5 μm, 2 μm, 1 μm or 500 nm. In somepreferred embodiments the separator structure, and hence gate length, isless than 1 μm; in general a lower step height facilitates good stepcoverage by the overlying layers. It will be appreciated that the gatelength (the source-drain gap) is substantially equal to the height ofthe separator structure (ignoring channel “end effects”).

The source, drain and gate electrodes may be deposited by a range oftechniques including, for example, physical vapour deposition. The gatemetal (and in later described embodiments the source and drain metal)may be patterned by conventional photolithographic techniques.Alternatively (but less preferably) a shadow mask may be employed.

The organic semiconductor and gate dielectric materials may be depositedby a range of techniques including solution processing techniques,including but not limited to, ink jet printing, spin coating (afterwardsremoving material from unwanted areas by plasma etching or laserablation), stamp contact, offset lithography, screen printing androll-printing (suitable printers for the latter for the deposition oforganic materials are available from Toppan Printing Co. Ltd of Tokyo,Japan).

In one preferred fabrication method the fabrication steps are asfollows:

1. Deposit and pattern the separator structure.

2. Evaporate source/drain metal and pattern to define source/drainelectrodes.

3. Deposit organic semiconductor (OTFT material) over the source anddrain electrodes and pattern if/as required.

4. Deposit gate dielectric and pattern if/as required.

5. Deposit and pattern gate metal.

Referring again to FIG. 1, it can be seen that the separator structure104 is undercut. This allows the source and drain metal to be depositedin a single (self-aligned/self-shadow masked) step in which theseparator structure prevents the source and drain electrodes from cominginto direct contact at the channel of the device.

To fabricate an undercut separator a variety of techniques may beemployed. Preferably a photodefinable polymer or photoresist such aspolyimide or an acrylic photoresist is lithographically patterned usinga mask or reticle and then developed to produce a desired channel-edgeface angle. Either a positive or a negative photoresist may be employed(for example there are image reversal methods which may be employed toreverse an image in a positive resist). To obtain an undercutphotoresist the photoresist may be under-(or over-) exposed andoverdeveloped; optionally an undercut profile may be assisted by soakingin a solvent prior to development. Rather than an edge face with auniform slope, the separator structure may also be etched to define anundercut shelf, for example by using a wet or dry isotropic etchprocess. The skilled person will be aware that there are many variationsof the basic spin, expose, bake, develop, and rinse procedure used inphotolithography (see, for Example, A. Reiser, Photoreactive Polymers,Wiley, New York, 1089, page 39, hereby incorporated by reference). Someparticularly suitable resist materials are available from ZeonCorporation of Japan, who supply materials adapted for the fabricationof organic electroluminescent displays (negative resist materials in theELX series, and positive resist materials in the WIX series).

Referring now to FIG. 2, this shows some alternative configurations forthe edge of the face of the separator structure adjacent the channel ofthe device. Thus it can be seen that in embodiments an undercut is notnecessary. In this case a small source-drain gap may be formed bydepositing the source-drain metal at an angle (or range of angles)beyond the rising slope of the separator structure. This has theadvantage of coating, and providing (electrical) continuity on theopposing slope, which might be helpful in some structures. Use of aseparator structure without an undercut is particularly convenient whenthe separator is not formed using conventional photolithography, forexample where the structure is formed by stamping where an undercutprofile can be difficult to achieve.

Referring now to FIGS. 3 a and 3 c, these show views from above of thesource and drain electrodes of a transistor constructed according to themethod of FIG. 1, illustrating the gate width (W) and gate length (L).As can be seen the gate length can be made very small and the gate widthcan be made large, particularly with the serpentine-type structure ofFIG. 3 c. This is advantageous because the source drain current in athin film transistor is proportional to the ratio W/L and hence bymaking W large and L small the source drain current can be increased fora given gate voltage or, similarly, a reduced gate voltage can beemployed. A further important advantage arising from the structure ofFIG. 1 is the very high uniformity of gate length which is achievable,because the uniformity of the gate length is determined by theuniformity of the layer thickness rather than by the lateral patterning(which is generally lower resolution).

Referring to FIG. 4, this shows a second example of an embodiment of athin film transistor according to the invention, in which like elementsto those of FIG. 1 are illustrated by like reference numerals. Thetransistor of FIG. 4 provides similar advantages to those describedabove but the fabrication technique and some elements of the structurediffer. In particular, the source and drain electrodes are formed inseparate steps, a layer of insulating material being deposited over thesubstrate and partially over an edge of the source electrode adjacentthe channel prior to deposition of the drain electrode. Because separatemetal layers are employed the source and drain may overlap slightly, forexample by less than 15 μm, 10 μm, 5 μm or 2 μm, although preferablythere is no overlap. The skilled person will nonetheless recognise thatcompared with the vertical device described in the Science papermentioned in the introduction there is substantially no overlap of thesource and drain electrodes. One advantage of the structure of FIG. 4 isthat, in operation, its behaviour is closer to a conventional, lateralthin film field effect transistor. One potential disadvantage is thatfabrication of the structure uses more steps than fabrication of thestructure of FIG. 1, although in practice this may not be a disadvantagesince additional steps may in any case be employed for fabrication ofother structures on the substrate, for example depending upon the trackrequirements of a design for an OLED display panel into which thetransistor is incorporated.

The examples of materials and layer thicknesses described above inrelation to FIG. 1 also apply to the FIG. 4 embodiment. However because(as described further below) the source electrode is patterned prior todeposition of the drain it may be convenient to purchase a substrate onwhich an electrode layer such as ITO (indium tin oxide) has already beendeposited in preparation for patterning to define the source electrodeand other connections.

As illustrated in FIG. 4, the channel-edge face of the layer ofinsulating material 104 makes a positive angle with the underlyingsubstrate (as opposed to the negative angle of the undercut shown inFIG. 1)—that is, it tapers towards the substrate—and this has theadvantage of facilitating coverage of the step.

In a example method of fabrication of the structure of FIG. 4, thefabrication steps are as follows.

1. Deposit and pattern first electrode (source, or drain).

2. Coat substrate with insulating material 104.

3. Deposit and pattern second electrode (source, or drain).

4. Etch away the insulating material 104 not protected by the secondelectrode metal, intentionally under-etching to leave a positive slope.

5. Deposit semiconductor (organic TFT) material (for example, by any ofthe methods described above) and pattern as/if necessary.

6. Deposit dielectric material and pattern as/if necessary.

7. Deposit and pattern gate electrode metal.

Depending upon the structural device in which the TFT is incorporated astep between steps 3 and 4 above may be included to add one or morevias.

Referring to FIGS. 3 b and 3 d, which in an analogous manner to FIGS. 3a and 3 c for FIG. 1, show a view from above of the source and drainelectrodes of the TFT structure of FIG. 4. It can be seen that, in thisexample, there is a slight overlap between the source and drainelectrodes.

Referring next to FIGS. 5 a and 5 b, this shows first and secondexamples of a double transistor structure based upon the structures ofFIGS. 1 and 4 respectively (like elements being indicated by likereference numerals). Broadly speaking the separator structure orinsulating layer 104 is provided with two channel-defining faces, forexample opposing one another. In embodiments of this double structurethe drain (or source) electrode 108, that is the electrode depositedover the separator structure/insulator 104, has no external connectionsbut comprises a common drain (or source) connection for the twotransistors. Although as illustrated in FIGS. 5 a and 5 b the gateconnections for the two transistors may be separate, in other preferredembodiments the gate metal is extended to provide a single, common gateconnection for both devices. In this way the structure may have acircuit as shown in FIG. 5 c comprising a pair of series-connected fieldeffect transistors with a common gate (control) connection. This has theadvantage of increased isolation when the devices are switched off (notselected).

A transistor of the type described above may be incorporated into anactive matrix electroluminescent display, in particular an OLED (organiclight emitting diode) display the transistor may be used to facilitate alarger drive current or lower control voltage and/or for a doublestructure, better isolation. Importantly, however, the above-describeddevices enable the fabrication of transistors with very uniform gatelength over the area of a display. Device uniformity presents aparticular problem in the context of displays because, unlike integratedcircuits in which as device size shrinks the overall area of the ICshrinks, in a display the tendency is for the overall area to stay thesame size or to increase whilst there is a desire to reduce the size ofdrive circuitry in order, for example, to increase aperture ratio.

Displays fabricated using OLEDs provide a number of advantages over LCDand other flat panel technologies. They are bright, colourful,fast-switching (compared to LCDs), provide a wide viewing angle and areeasy and cheap to fabricate on a variety of substrates. Organic (whichhere includes organometallic) LEDs may be fabricated using materialsincluding polymers, small molecules and dendrimers, in a range ofcolours which depend upon the materials employed. Examples ofpolymer-based organic LEDs are described in WO 90/13148, WO 95/06400 andWO 99/48160; examples of dendrimer-based materials are described in WO99/21935 and WO 02/067343; and examples of so called small moleculebased devices are described in U.S. Pat. No. 4,539,507.

A typical OLED device comprises two layers of organic material, one ofwhich is a layer of light emitting material such as a light emittingpolymer (LEP), oligomer or a light emitting low molecular weightmaterial, and the other of which is a layer of a hole transportingmaterial such as a polythiophene derivative or a polyaniline derivative.

Organic LEDs may be deposited on a substrate in a matrix of pixels toform a single or multi-colour pixellated display. A multicoloureddisplay may be constructed using groups of red, green, and blue emittingpixels. So-called active matrix (AM) displays have a memory element,typically a storage capacitor and a transistor, such as described above,associated with each pixel. In a bottom-emitting display light isemitted through the substrate on which the active matrix circuitry isfabricated; in a top-emitting display light is emitted towards a frontface of the display so avoiding the active matrix circuitry (an exampleis described in WO 2005/071771, incorporated by reference). Examples ofpolymer and small-molecule active matrix display drivers can be found inWO 99/42983 and EP 0,717,446A respectively (also incorporated byreference).

FIG. 6 a schematically illustrates a bottom-emitting OLED display 600respectively in which substrate 102 bears an active matrix drivercircuit 650 for each pixel, over which is provided an OLED pixel 614.FIG. 6 b shows details of an example OLED structure comprising an anodelayer 606 such as ITO, over which one or more layers of OLED material608 are deposited in wells defined by banks 612, for example by spincoating and subsequent patterning, or by selective deposition using aninkjet-based deposition process (see, for example, EPO 880 303 orWO2005/076386). In the case of a polymer-based OLED layers 608 comprisea hole transport layer 608 a and a light emitting polymer (LEP)electroluminescent layer 608 b. The electroluminescent layer maycomprise, for example, PPV (poly(p-phenylenevinylene)) and the holetransport layer, which helps match the hole energy levels of the anodelayer and of the electroluminescent layer, may comprise, for example,PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). Amultilayer cathode 610 overlies the OLED material 608 and typicallycomprises a low work function metal such as calcium (optionally withlayer of material such as barium fluoride adjacent the LEP for improvedelectron energy level matching) covered with a thicker, capping layer ofaluminium (in top-emitters the cathode layer is kept sufficiently thinto be substantially transparent). Mutual electrical isolation of cathodelines may be achieved or enhanced through the use of cathode separatorssimilar to separator structure 104 (not shown in the Figure).

FIG. 6 c, which is taken from our application WO03/038790, shows anexample of a current-controlled active matrix pixel driver circuit 650.In this circuit the current through an OLED 652 is set by usingtransistors 656 a,b to set a drain source current for OLED drivertransistor 658 (using a reference current sink 654) and to memorise thedriver transistor gate voltage required for this drain-source current ona capacitor 660. Thus the brightness of OLED 652 is determined by thecurrent, I_(col), flowing into reference current sink, which ispreferably adjustable and set as desired for the pixel being addressed.In addition, a further switching transistor 664 is connected betweendrive transistor 658 and OLED 652. In general one current sink isprovided for each column data line.

In this example driver circuit transistor 658 may have a structure asdescribed above for increased drain-source current for a given gatevoltage. Transistors 656 b and 664 may have a structure as describedabove for similar reasons and, additionally, may be fabricated as adouble device of the general type shown in FIG. 5 for improvedisolation. Transistor 656 a may have a structure as described above forimproved isolation.

FIG. 7 shows a view from above of a portion of an active matrix OLEDdisplay 700 which may advantageously incorporate a transistor structureas described above. Like elements to those described above are describedby like reference numerals.

Although some preferred embodiments of the above-described structureshave been described with specific reference to the fabrication of a thinfilm transistor, in particular a field-effect transistor, the skilledperson will understand that the above-described structures may also beemployed to fabricate other types of electronic device including, butnot limited to, a diode, thyristor and the like. The skilled person willalso recognise that in the above-described embodiments of the TFTstructures the labelling of the source and drain electrodes may beexchanged so that the electrode on the separator structure or insulatormay be, for example, the source rather than the drain electrode.Likewise enhancement or depletion mode devices may be fabricated.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

The invention claimed is:
 1. A method of fabricating an organicelectronic device on a substrate, the method comprising: providing saidsubstrate with a first electrode for said organic electronic device;forming a separator structure on said substrate; forming a secondelectrode for said organic electronic device over said separatorstructure; and depositing a layer of organic semiconducting materialover said first and second electrodes; and wherein said separatorstructure comprises a layer of insulating material, and wherein saidforming of said separator structure comprises forming said secondelectrode over said layer of insulating material and then patterningsaid layer of insulating material using said second electrode as an etchmask and/or photomask.
 2. A method as claimed in claim 1 wherein saidforming of said separator structure comprises forming said structuresuch that it partially overlaps said edge of said first electrode.
 3. Amethod of forming an organic thin film transistor (TFT) comprisingemploying the method of claim 1 to fabricate a source electrode and adrain electrode of said transistor, followed by deposition of adielectric layer and deposition of a gate layer of said transistor.
 4. Amethod of forming a double TFT structure using the method of claim 3,wherein said second electrode forming comprises forming a common sourceor drain electrode for said double TFT structure.
 5. A method offabricating an organic thin film transistor on a substrate the methodcomprising: depositing a first layer of conductive material on saidsubstrate and patterning said first layer of conductive material todefine a first source or drain electrode of said transistor; depositinga layer of insulating material on said substrate over said firstelectrode; depositing a second layer of conductive material over saidlayer of insulating material and patterning said second layer ofconductive material to define a second, drain or source electrode ofsaid transistor; patterning said layer of insulating material using saidsecond electrode as an etchmask and/or photomask to expose at least partof said first electrode; depositing organic semiconducting material oversaid first and second electrodes; depositing gate dielectric materialover said organic semiconducting material; and depositing conductivematerial over said dielectric material to provide a gate electrode forsaid transistor.
 6. A method of fabricating an organic thin filmtransistor on a substrate the method comprising: depositing a separatorstructure on said substrate; patterning said separator structure todefine a source-drain separator; depositing conductive material on saidsubstrate bearing said source-drain separator to define source and drainelectrodes of said transistor, said source electrode having an edgefacing said drain electrode, said drain electrode having an edge facingsaid source electrode; depositing organic semiconducting material oversaid source and drain electrodes to provide a conducting channel betweensaid source and drain electrodes; depositing dielectric material oversaid organic semiconducting material; and depositing conductive materialover said dielectric material to provide a gate electrode for saidtransistor, wherein the source-drain separator has an undercut edge,wherein one of the source and drain electrodes is formed over theseparator structure at a first height, and the other of the source anddrain electrodes is formed over the substrate at a second, lower height,wherein said channel is formed between said edges of the source anddrain electrodes, and wherein substantially all said source electrode islaterally positioned to one side of said channel and substantially allsaid drain electrode is laterally positioned to the other side of saidchannel.
 7. A method according to claim 6 wherein the source and drainelectrodes are separated by a distance of less than 10 μm.
 8. A methodaccording to claim 6 wherein the separator structure is formed fromphotoresist material, and the separator structure is patterned byphotopatterning to form the source-drain separator.
 9. A methodaccording to claim 6 wherein at least one of the organic semiconductingmaterial, the dielectric material, and the gate electrode are depositedfrom a solution in a solvent.
 10. A method according to claim 6 whereinthe conductive material forming the source and drain electrodes isdeposited by evaporation or sputtering.
 11. A method according to claim10 wherein said forming of said source and drain electrodes comprisesdirectional deposition of electrode material at an angle such that ashadow of an edge of said separator structure defines an electrode edge.12. A method according to claim 6 wherein the conductive materialforming the source and drain electrodes comprises a metallic element.13. A method according to claim 12 wherein the conductive materialforming the source and drain electrodes consists essentially of anelemental metal or an alloy thereof.
 14. An organic electronic device,the device comprising: a substrate supporting a first electrode; aseparator structure over said substrate; a second electrode over saidseparator structure and at a height above said first electrode, saidfirst electrode having an edge facing said second electrode, said secondelectrode having an edge facing said first electrode; and a layer oforganic semiconducting material over said first and second electrodes toprovide a conducting channel between said first and second electrodes;and wherein said channel is formed between said edges of the first andsecond electrodes, wherein substantially all said first electrode islaterally positioned to one side of said channel and substantially allsaid second electrode is laterally positioned to the other side of saidchannel, wherein the separator structure has an undercut edge.
 15. Anorganic electronic device as claimed in claim 14 wherein said separatorstructure comprises a layer of electrically insulating material.
 16. Anorganic electronic device as claimed in claim 14 wherein said devicecomprises a transistor, and wherein one of said first and secondelectrodes comprises a source electrode of said transistor and the othera drain electrode of said transistor, the device further comprising alayer of gate dielectric over said layer of organic semiconductingmaterial and a gate electrode over said layer of gate dielectric.
 17. Anorganic electronic device as claimed in claim 16 wherein said gateelectrode overlies said source electrode without an interveningconducting layer and wherein said gate electrode overlies said drainelectrode without an intervening conducting layer.
 18. A doubletransistor structure comprising two transistors each as claimed in claim16, and wherein one of said source and drain electrodes is sharedbetween said two transistors.
 19. A double transistor structure asclaimed in claim 18 wherein said double transistor structure lacks anexternal connection to the said shared electrode.
 20. An organicelectronic device, the device comprising: a substrate; a first electrodeover said substrate at a first height above said substrate; a separatorstructure over said substrate; a second electrode over said separatorstructure and at a second height, greater than said first height, abovesaid substrate, said first electrode having an edge facing said secondelectrode, said second electrode having an edge facing said firstelectrode; and a layer of organic semiconducting material over saidfirst and second electrodes to provide a conducting channel between saidfirst and second electrodes; and wherein said channel is formed betweensaid edges of the first and second electrodes, wherein the separatorstructure has an undercut edge and there is substantially no lateraloverlap between said first and second electrodes.